1. Field of the Invention
The present invention relates to color cathode ray tubes, and more particularly, to a tension mask frame assembly for a color cathode ray tube, having an improved tension mask which is fixed to a frame.
2. Description of the Related Art
In color cathode ray tubes (color CRTs), an electron beam emitted from an electron gun lands on a fluorescent film through electron beam passing holes in a shadow mask and excites the fluorescent film to form an image.
The screen surface of conventional color CRTs which form an image as described above is designed so as to have a predetermined curvature in consideration of the deflection trajectory of an electron beam which is emitted from an electron gun and deflected by a deflection yoke. The tension mask is designed so as to have a curvature corresponding to the curvature of the screen surface.
A shadow mask, which is manufactured so as to have a curvature similar to the curvature of the inner surface of the screen surface, is heated by an electron beam, that is, by a thermoelectron, emitted from the electron gun, which causes a doming phenomenon in which the shadow mask is swollen toward a panel. The doming phenomenon prevents the electron beam from accurately landing on the fluorescent film. As described above, the screen surface is designed to have a predetermined curvature, such that the view angle is narrowed and the fluorescent film is excited at the periphery of the screen surface, thus distorting the formed image.
In order to solve these problems, a color cathode ray tube (CRT) having a flat-surface screen has been developed. In such a color CRT, a tension mask, in a state where a tensile force is applied thereto, is fixed to the inner surface of a panel so as to be separated by a predetermined distance from a fluorescent film formed on the inner surface of the panel. In this state, the panel is sealed with a funnel on which an electron gun and a deflection yoke are mounted.
Examples of a tension mask installed in a color CRT are respectively disclosed in U.S. Pat. No. 5,488,263, U.S. Pat. No. 4,973,283, U.S. Pat. No. 4,942,332, U.S. Pat. No. 4,926,089 and U.S. Pat. No. 6,097,142, for example.
An example of a tension mask, illustrative and exemplary of those disclosed in the aforementioned patents, is shown in FIG. 1. As shown in FIG. 1, the tension mask 20 has a plurality of strips 21 formed in parallel, and a slot 23 is formed by strips 21 and tie bars 22 having a vertical pitch PV, which connect the strips 21 to each other. Here, the vertical pitch PV of the tie bar 22 and the horizontal pitch PH of slots 23 are equal at the center portion of the tension mask 20 to those at the peripheral portion of the tension mask 20. The slots 23 have a plurality of auxiliary tie bars 24 which extend from a strip 21 on one side to an opposite strip side.
However, in a tension mask 20 having the auxiliary tie bars 24 as described above with respect to FIG. 1, as the vertical pitch PV of the tie bar 22 is relatively increased, a ligament ratio is correspondingly lowered. That is, referring to FIG. 2, the ligament ratio obtained by dividing the width W of the tie bar 22 by one of two equal parts PV into which the vertical pitch of a slot is divided. Thus, as the vertical pitch of a slot increases, the ligament ratio is relatively lowered.
As described above, when the ligament ratio is lowered, a supporting force between strips 21 is typically deteriorated, so that the tension mask 20 can be easily plastic-deformed by an impact applied from an external source, such as an impact applied in a vertical direction. That is, referring to FIGS. 1 and 2, a vibration, which is transmitted from the center to the periphery of the tension mask 20 when an impact is applied in the vertical direction of the tension mask 20, can cause a sudden increase in stiffness at a relatively-wide end strip area, which is the horizontal end of the tension mask 20, so that the edge of the tension mask is 20 plastic-deformed. This phenomenon occurs since an impact applied to the center portion is transmitted to the horizontal edge without reduction due to the fact that the vertical pitch of the tension mask 20 is the same at the center portion and the peripheral portion.
U.S. Pa. No. 4,926,089 to Moore, entitled Tied Slit Foil Shadow Mask With False Ties, discloses a front assembly for a color cathode ray tube that includes a glass faceplate that has on its inner surface a centrally disposed phosphor screen. A metal foil shadow mask is mounted in tension on a mask support structure located on opposed sides of the screen. The mask includes a series of parallel strips separated by slits, the strips being coupled by widely spaced ties. The mask has, between the strips, one or more false ties extending partially between, but not interconnecting, adjacent strips. The screen may also have spaced ties interconnecting the grille lines with a periodicity much smaller than that of the mask ties and below an observer""s resolution threshold at normal viewing distances.
U.S. Pat. No. 4,942,332 to Adler et al., entitled Tied Slit Mask For Color Cathode Ray Tubes, discloses a slit-type foil tension mask and associated front assembly for a color cathode ray tube that includes a series of parallel strips separated by slits. The strips are loosely coupled by widely spaced ties, the wide tie spacing being such as to produce a strip coupling which promotes handleability of the mask during mask and tube fabrication, and which facilitates damping of strip vibration when mounted in a tube. Also, in FIG. 11 therein, it is disclosed that the vertical position, or pitch, of the ties is not constant but is randomly varied from tie to tie to suppress tie visibility. Also, in FIG. 12 therein, it is disclosed that false ties are placed along the slit edges at regular intervals between the real ties and with a pitch less than that of the real ties.
U.S. Pat. No. 4,942,333 to Knox, entitled Shadow Mask With Border Pattern, discloses a shadow mask adapted for tensioned mounting in a flat faced color CRT having a pattern of slits in the border regions of the mask disclosed to provide uniform distribution of tensile stresses across the mask when mounted in the CRT.
U.S. Pat. No. 4,973,283 to Adler et al., entitled Method Of Manufacturing A Tied Slit Mask CRT, discloses a slit-type foil tension mask and associated front assembly for a color cathode ray tube including parallel strips separated by slits. The strips are loosely coupled by widely spaced ties, the wide tie spacing being such as to produce a strip coupling which promotes handleability of the mask during mask and tube fabrication, and which facilitates damping of strip vibration when mounted in a tube.
U.S. Pat. No. 5,072,150 to Lee, entitled Shadow Mask Assembly for Color Picture Tube, discloses a shadow mask frame for a color picture tube that has side walls which are cut out to form cut-out sections, leaving only a plurality of bridge portions. A separate supporting means for the frame is provided in direct contact with the shadow mask.
U.S. Pat. No. 5,126,624 to Ji, entitled Color Cathode Ray Tube Having Improved Spring Type Contactor, discloses a color cathode ray tube having a spring type contactor. The spring type contactor effects electrical connection between a frame and a conductive coating deposited on the inner surface of the funnel, and comprises an xe2x80x98.OMEGA.xe2x80x99 shaped fitting portion for being inserted into holes respectively perforated on the shield and the frame so as to be locked therein, a pair of legs abutting the edge of the hole of the shield, and a xe2x80x98Cxe2x80x99 shaped contact portion extending from one of the legs to contact the conductive coating on the inner surface of the funnel.
U.S. Pat. No. 5,210,459 to Lee, entitled Shadow Mask Structure Of A Color Cathode Ray Tube, discloses a cathode ray tube with a shadow mask, the shadow mask structure being suspended and fixed behind the panel of the cathode ray tube. Plate springs for connecting the shadow mask structure and the panel are placed so as to apply pulling forces at either the sides or the corners of the shadow mask frame, and so as to hold the shadow mask to the skirt so as not to deform the shadow mask.
U.S. Pat. No. 5,488,263 to Takemura et al., entitled Color Selecting Electrode For Cathode-Ray Tube, discloses a color selecting electrode for use in a cathode-ray tube which includes a frame having a pair of opposed first supports and a pair of opposed second supports extending in a direction so as to cross the pair of first supports, and grid elements disposed on the pair of first supports at a fixed pitch and stretchedly bridging the pair of first supports.
U.S. Pat. No. 5,523,647 to Kawamura et al,. entitled Color Cathode Ray Tube Having Improved Slot Type Shadow Mask, discloses a color cathode ray tube having a slot type shadow mask. The shadow mask assembly is suspended inside the panel, and is disclosed as including a mask frame, and the shadow mask held on the mask frame, the shadow mask having a large number of grilles and bridges disposed at an interval for connecting adjacent grilles, the grilles and the bridges having sections which are concave in opposite directions, respectively.
U.S. Pat. No. 5,534.746 to Marks et al., entitled Color Picture Tube Having Shadow Mask With Improved Aperture Spacing, discloses a color picture tube that includes a shadow mask and a dot screen, wherein the mask is rectangular and has two horizontal long sides and two vertical short sides. The long sides are parallel to a central major axis of the mask and the short sides are parallel to a central minor axis of the mask. The mask includes an array of apertures arranged in vertical columns and horizontal rows. Apertures in one row are disclosed as being in different columns than are the apertures in adjacent rows. The vertical spacing between apertures in the same column is the vertical pitch of the apertures, and the horizontal spacing between apertures in the same row is the horizontal pitch of the apertures. It is disclosed that the horizontal pitch of the apertures increases from the minor axis to the short side of the masks and decreases from the major axis to the long sides of the mask. Also, along the major axis, the vertical pitch of the mask is disclosed as decreasing from the center to the short sides of the mask and, adjacent the long sides of the mask, it is disclosed as increasing from the minor axis to the corners of the mask.
U.S. Pat. No. 6,057,640 to Aibara, entitled Shadow Mask For Color Cathode Ray Tube With Slots Sized to Improve Mechanical Strength And Brightness, discloses a shadow mask for a cathode ray tube, including a plate having a first surface and a second surface. The plate is formed with at least one line of slots between which bridge portions are formed, each slot being spaced away from adjacent slots by a predetermined pitch. The bridge portions are defined by a first length at the first surface of the plate and a second length at the second surface of the plate, the first and second lengths being determined so that a factor is in the range of 5% to 15%, the factor being defined as a ratio of the smaller of the first and second lengths to the predetermined pitch.
U.S. Pat. No. 6,072,270 to Hu et al., entitled Shadow Mask For Color CRT, discloses a shadow mask employed as a color selection electrode in a multi-electron beam color cathode ray tube (CRT), the surface area of the mask being reduced by increasing the length of the individual elongated beam passing apertures, or slots, while-reducing the ratio of the width of the bridge portion of the mask between adjacent apertures to the length of the aperture.
U.S. Pat. No. 6,097,142 to Ko, entitled Shadow Mask Having An Effective Face Area And Ineffective Face Area, discloses a shadow mask including an effective face area constituting a central portion of the shadow mask. The effective face area has electron beam apertures, which electrons pass through. A secondary ineffective face area surrounds the effective face area and also has apertures. A frame attaching border further surrounds the secondary ineffective face area, and a primary ineffective face area at least partially surrounds the frame attaching border. Corners of the shadow are adjacent to the primary ineffective face area and do not have apertures. It is disclosed that portions of the primary and/or secondary ineffective areas are treated with tie bar grading and/or have round corners.
To promote resolving the above problem, an objective, among other objectives, of the present invention is to provide a tension mask frame assembly for a color cathode ray tube, by which a tension mask is prevented from being plastic-deformed by a tensile force applied to the tension mask or by a strong impact applied from an external source.
To achieve the above objective and other objectives of the present invention, the present invention provides a tension mask frame assembly for a color cathode ray tube including: a tension mask having a plurality of strips on which slots are formed, the slots being separated by a predetermined distance from each other on a thin plate, and real bridges for partitioning slots at a predetermined pitch interval by connecting adjacent ones of the plurality of strips to each other; and a frame which supports the corresponding edges of the tension mask, whereby the vertical pitch of the real bridges becomes smaller, such as in a stepwise relation, in a direction from the center portion of the tension mask to the peripheral portion of the tension mask, with a vertical pitch of the plurality of real bridges in the center portion of the tension mask being greater than a vertical pitch of the plurality of real bridges in a peripheral portion of the tension mask.
Also, in the present invention, the tension mask desirably includes a dummy bridge that extends from a strip on at least one side of a corresponding slot to a strip on the opposite side of the corresponding slot, the dummy bridge being formed on a slot partitioned by a corresponding one of the real bridges.
Also, to achieve the above objective and other objectives of the present invention, the present invention provides a tension mask frame assembly for a color cathode ray tube including: a tension mask having a plurality of strips on which slots are formed, the slots being separated by a predetermined distance from each other on a thin plate, and real bridges for partitioning slots at a predetermined pitch interval by connecting adjacent ones of the plurality of strips to each other; and a frame which supports the corresponding edges of the tension mask, whereby a tensile force is applied to the tension mask, and the vertical pitch of the real bridges becomes smaller at both shorter sides of the tension mask than at the center portion of the tension mask.